1. Field
Exemplary embodiments of the present invention relate to a semiconductor device fabrication technology, and more particularly, to a method for forming a polysilicon layer and a method for forming buried bit lines.
2. Description of the Related Art
When buried bit lines BBL are used in cells of a vertical transistor structure, two cells may neighbor one buried bit line. For example, for a cell to be driven by a buried bit line, a One-Side Contact (OSC) process in which a contact is formed in any one side of an active region while the other side of the active region is insulated.
In a cell having a vertical transistor structure formed through the one-side contact process, an active region includes a body isolated by trenches and a pillar formed over the body. The buried bit lines BBL fill the trenches between bodies and word lines (or vertical gates) are disposed adjacent to the sidewalls of pillars and extended in a direction crossing the buried bit lines BBL. Channels are formed in a vertical direction using the word lines.
According to the one-side contact process, a portion of any one sidewall of a body, which is an active region, has to be exposed for connection between the active region and a buried bit line BBL. To form the one-side contact, the trenches between the bodies are filled with a gap-fill layer having a step height. As for the gap-fill layer, a polysilicon layer may be used.
However, a seam may occur during the formation of the polysilicon layer due to narrow linewidth of the contact. The occurrence of the seam causes lack of uniformity during an etch-back process where not only a liner oxide layer which is formed for insulation from the buried bit lines BBL but also the substrate under the trenches are damaged and cause formation of active punch in the substrate.
FIGS. 1A and 1B are Transmission Electron Microscopic (TEM) photographs illustrating results of the conventional technology.
FIG. 1A illustrates that the middle of the polysilicon layer is hollow due to occurrences of seam.
FIG. 1B illustrates that the liner oxide layer and the substrate under the trenches are damaged due to the occurrence of seam on the polysilicon layer in FIG. 1A and that an active punch is formed thereby.
Therefore, when the polysilicon layer is formed, the polysilicon layer should be formed without a defect such as a seam.